Uninterruptible power supplies or systems (commonly referred to as UPS) are used to provide back-up power to critical loads, such as computer systems, where a loss of line power can result in the interruption of programs and the loss of valuable data. Uninterruptible power supplies may also provide a line power conditioning function to ensure that transient spikes, low-voltage conditions, or distorted power waveforms on the AC power system do not disturb the operation of the computer which is supplied with power through the UPS. Typically, the UPS includes a battery which is interfaced through an inverter to the AC output line of the system at the same frequency and with substantially the same waveform as the normal AC power input to the system. It is desirable that the switching between line and battery power at the time of a fault is accomplished as smoothly as possible so that substantial transient spikes or dips in the waveform supplied to the AC output line do not occur.
In certain UPS systems power is normally delivered from the AC power mains through a transformer to the load. Upon detection of a power outage or brown-out (low voltage) on the AC power lines, a switch is opened to disconnect the primary of the transformer from the AC power mains. Simultaneously, an inverter is turned on to supply power from a battery to an auxiliary primary of the transformer which then takes over the job of supplying the power to the load. Such a UPS is shown in U.S. Pat. No. 5,315,533 to Stich, et al., entitled Back-Up Uninterruptible Power System. UPS using a ferroresonant transformer are shown in U.S. Pat. No. 4,692,854 to Richard V. Baxter, et al. entitled Method and Apparatus for Modulating Inverter Pulse Width and U.S. Pat. No. 5,182,518 to Stich, et al. entitled Inverter and Battery Testing for Uninterruptible Power Systems.
In uninterruptible power systems it is desirable to be able to regulate the output voltage over a wide range of AC input voltages without having to switch to inverter operation. For example, in a brown-out situation the AC input voltage may drop so as to cause AC output to fall below acceptable levels, while still providing some power to the load. One way to regulate such a voltage drop, without resorting to inverter operation, is to provide for multiple taps to the transformer primary connected to the AC power input. Each of the winding taps is provided with a tap changing switch. By connecting the AC input to the transformer through a selected one of the tap changing switches the UPS can regulate the output voltage despite swings in the input voltage without resorting to inverter operation. To provide relatively fine regulation over a large range of varying input voltages generally has required a large number of taps, tap switches, and associated switch control circuitry. By minimizing inverter operation, however, drain on the battery can be minimized, thereby increasing battery life.
In UPS systems of this type the transfer to battery power is initiated by a determination of the quality of the AC power system voltage. To achieve this determination the incoming line voltage waveform is usually monitored. When the line voltage is determined to be defective--that is, the line voltage is excessively noisy, the waveform is distorted, or power failure is complete--, back-up power is automatically supplied to the connected load by an inverter which is powered from the UPS batteries. Very often, especially in the case where primary AC power is derived from an AC generator, the incoming line voltage waveform is badly distorted, noisy, or unstable, resulting in many forced transfers from utility to battery power. However, many of these transfers may be initiated by poor wave shape or other non-critical anomalies which occur even when the incoming power waveform is adequate to power the loads without interruption. These often unnecessary transfers are undesirable and tend to deplete the batteries, so that when there is an actual power outage the batteries are not charged sufficiently to assure reliable back-up power for the designed back-up time. It is therefore desirable that the transfer mechanism be desensitized from those line distortions which are not indicative of power line failure. One method to overcome this problem is described in U.S. Pat. No. 5,229,651, to Baxter, Jr., et al., entitled Method and Apparatus for Line Power Monitoring for Uninterruptible
Power Supplies. By this method a reference waveform, which is a composite of data from prior waveform cycles, is generated and is used in comparison with the incoming waveform to detect line faults. In U.S. Pat. No. 5,315,533 the comparison of the two waveforms is accomplished at high speed using a comparator circuit, and a selected allowable tolerance between the reference and the line voltage--a "line delta"--is employed to minimize unnecessary transfers to battery power.
A similar problem with noise arises at line voltage zero crossings. The line voltage zero cross signal is used by a UPS system for a variety of purposes, principal among which are system synchronization and line frequency determination. Line voltage waveform distortion and noise near the line zero cross can give rise to false zero cross indications. It is therefore desirable that a UPS system be able to distinguish a true line voltage zero cross from those resulting from waveform distortions. A typical solution in the prior art has involved the simple filtering of the line signal.
In UPS systems, batteries are one of the major causes of system failure. Battery performance can deteriorate due to the natural aging of the battery, and performance deterioration can be accelerated by using improper charging techniques, operating at excessive temperature, and allowing batteries to discharge below proper cell voltage for a long period. The latter condition leads to battery sulfation, which in some cases can be severe enough that the full battery capacity cannot be recovered.
For optimum charging it is desirable that the battery charger be able to charge in various modes. For example, for a heavily discharged battery, sulfation may have occurred and it is then necessary not only to charge the battery but also to equalize the cells, i.e., bring all cells to the same voltage and to desulfate these cells. This requires a controlled over-charge of the battery. It has also been determined, from automotive battery research, that for optimum battery life the batteries should be allowed to sit idle for much of their life. This requires a charger which will not float, or trickle charge, a battery to maintain the battery charge level once that proper level is reached. Furthermore, overcharging should be avoided as this can damage the battery. Battery chargers which can operate in multiple modes are known. What is desirable, however, is a battery charger with multiple modes of operation which allows for the charge profile to be easily tailored to the recommendations of specific battery manufacturers or to a user's preferred method.